The widespread use of solvents in industrial applications has resulted in increased emissions of volatile organic compounds (VOCs) into the atmosphere, giving rise to environmental concerns and prompting stricter legislative controls on such emissions. As a consequence, manufacturers of pharmaceuticals, coated products, textiles, and polymer composites and foams, as well as hydrocarbon producers and distributors, face a dilemma in removing VOCs from process gas streams in that, owing to rising energy prices, recovery costs are very often higher than the value of the VOCs recovered, even in light of rising solvent prices. This dilemma has led to inquires into more profitable methods of recovering condensable organic vapors from process gas streams.
One such method is disclosed in U.S. Pat. No. 4,480,393. By first concentrating the condensable vapors contained in a process gas stream carrier and then employing refrigeration condensation to effect their recovery, the method operates more energy efficiently than those methods previously known. Particularly, this method entails first passing a process gas stream carrying a condensable organic compound such as a solvent through, for example, a packed carbon bed which adsorbs the solvent vapor such that solvent is accumulated in the bed and a solvent-free process gas stream may be exhausted or recycled back to the process. Upon becoming saturated with adsorbed solvent, the bed is regenerated by the circulation of a heated inert gas stream therethrough to vaporize the solvent. Once vaporized, the solvent is carried by the inert gas stream from the bed to a refrigeration/condensation recovery system. Therein, the vapor-laden inert gas stream is first compressed and then passed through a turbine expander to thereby effect an expansive cooling at pressures near atmospheric to temperatures generally well below the boiling points of the solvents to be recovered. Separation of the condensable solvent vapors from the relatively non-condensable inert gas stream carrier may then be effected with the recycling of recovered solvent back to the process and the recycling of inert gas back to the packed bed to continue its regeneration.
As mentioned, this method advantageously operates more energy efficiently than methods previously known. By using a turbine expander coupled to a compressor to effect the expansive cooling of the solvent-containing inert gas stream effluent from the packed carbon bed regeneration, the energy liberated from the cooling of the gas stream in the expander can be used to drive the compressor. The compressor effects in the gas stream a pressurization that results in a better solvent condensation for a given operating temperature. In addition, an energy exchange can be effected between the hot solvent-containing inert gas stream effluent from the bed and the cooled inert gas stream recycled from the condensation separation to pre-heat the inert gas entering the compressor and to pre-cool the solvent-containing inert gas entering the turbine. The net effect of the foregoing is a method which enables solvent-intensive industries to comply with governmental regulations while maintaining economically profitable operations.
Attendant with industrial acceptance of the aforementioned method for VOC recovery have come demands for its further refinement. Namely, because the compressor side of the turbine compressor-expander is employed to pressurize the solvent-laden inert gas effluent from the bed, the heat of compression thereby imparted to the stream must be extracted therefrom prior to its expansive cooling in the expander side in order that a sufficiently low refrigeration temperature may be achieved. Although some of this waste heat may be recovered and used to heat the inert gas stream being recycled back to the bed to continue regeneration, this recovery necessitates the introduction of an additional heat exchanger into the process. Moreover, because the compressed solvent-laden stream is at a relatively high pressure as compared to the expansively cooled inert gas recycle stream, the pressure differential therebetween presents problems in heat exchanger design, construction and operation. In addition, because the inert gas recycle stream may be heated insufficiently to effect solvent vaporization within the bed solely by this heat exchange or by a subsequent heat exchange with the solvent-laden inert gas effluent from the bed, a heater must often be employed to supply the additional energy needed. Inasmuch as the capital expense associated the purchase of heat exchangers, heaters and the like, especially in light of attendant operational, energy, and maintenance expenses, is often seen as an obstacle to the adoption of this process by an industry so considering, there remains a need for further improvements in the methods for recovering VOCs from process gas streams.